1. Field of the Invention
Exemplary embodiments of the present invention relates to a signal receiving apparatus and method for a wireless communication system; and, more particularly, to a signal receiving apparatus and method for a wireless communication system using multiple antennas.
2. Description of Related Art
While the existing wireless communication systems have mainly provided voice services, the recent wireless communication systems increasingly tend to provide data services as well as voice services. In addition, the mainstream of data networks for providing data services has been cable-based data networks. However, many efforts to provide wireless services have been continuously made in order to ensure portability which is one of human being's basic desires. Thus, a variety of standards for high-speed wireless data transmission have been established and are now commercially available.
Examples of such wireless standards include wireless local area network (LAN), Wibro, WiMax, and so on. To meet the demands on a large amount of multimedia contents increasing in such wireless communication standards, advanced wireless transmission technologies have been used. Due to the use of such technologies, a data service supporting portability can be provided, and a video call and a large amount of multimedia contents can be used. Furthermore, examples of wireless communication technologies for high-speed data transmission include an Orthogonal Frequency Division Multiplexing (hereinafter, referred to as “OFDM”) scheme, a Multi Input Multi Output (hereinafter, referred to as “MIMO”) technology, and so on.
The OFDM scheme is to transmit a data stream not over a single carrier but over multiple subcarriers. In wireless communication channel environments, obstacles such as building cause multipaths, and the multipaths cause delay spread. When a transmission time of a next symbol is longer than a delay spread time, inter symbol interference (ISI) occurs. In this case, when viewed in a frequency domain, a frequency selective fading occurs selectively. When a single carrier frequency is used, an equalizer is used to remove ISI components. However, as the data rate increases, the complexity of the equalizer also increases.
In the OFDM scheme, data are transmitted in parallel at high speed over multiple subcarriers. Hence, a frequency selective fading of one subcarrier among the respective subdcarriers may be solved. Furthermore, in order to prevent the distortion of the orthogonal component due to ISI caused by a wave delayed during transmission a guard interval is inserted to solve the interference. The OFDM scheme is widely used as a core technology in many fields, for example, 802.11 Wireless LAN, Digital Multimedia Broadcasting (DMB), Power Line Communication (PLC), xDSL, 4G mobile communication, and High-Speed Portable Internet (HPi).
Moreover, the existing wireless communication systems have mainly provided voice services, and channel coding has been widely used in order to overcome poor channel properties. However, as high-quality multimedia services enabling an anytime, anywhere calling have been demanded, the trend is moving from voice services to data services. To this end, there is a need for technology which transmits a large amount of data at faster speed and with less error. However, the mobile communication environment greatly distorts signals due to multipath, shadow effect, propagation attenuation, interference, and the like. Specifically, a fading phenomenon due to multipaths causes serious distortion of a signal given by combination of signals which have different magnitudes and phases through different paths. Such a fading phenomenon is one of problems which must be solved in high-speed digital communications. One of methods for solving such a problem is a MIMO system.
A MIMO system is an advanced version of a Single Input Single Output (hereinafter, referred to as “SISO”) and use multiple antennas at a transmitting side and a receiving side. The MIMO system transmits and receives a plurality of signals through a plurality of antennas at a time. Hence, compared with an existing system, the MIMO system has an advantage that transmits a larger amount of data without increasing a bandwidth.
However, the MIMO system has a drawback in that it is vulnerable to ISI and frequency selective fading. To solve such a drawback, the MIMO system adopts the OFDM scheme. The OFDM scheme processes data in parallel, divides a high-speed data stream into a low-speed data stream, and simultaneously transmits the low-speed data stream over multiple carriers. Since low-speed parallel carriers are used, a symbol interval is increased and ISI is reduced. Furthermore, since a guard interval is used, ISI is removed almost completely. Moreover, since multiple carriers are used, the OFDM scheme is robust against a frequency selective fading. Consequently, by combining the two systems, the advantage of the MIMO system is used as it is, and the drawback of the MIMO system is eliminated using the OFDM system. A general type of a MIMO system uses N transmit (TX) antennas and N receive (RX) antennas.
For example, while an IEEE 802.11b wireless LAN system uses a complementary code keying (CCK) scheme and has a data rate of 11 Mbps, an IEEE 802.11g/a wireless LAN system adopting an OFDM scheme supports up to 54 Mbps, and an IEEE 802.11n wireless LAN system adopting a multiple antenna technology supports a physical layer data rate higher than 300 Mbps.
In addition, the most important consideration in the design of a wireless communication technology is the supportable throughput and the signal arrival distance. Although the IEEE 802.11n system provides a physical layer data rate of 300 Mbps, the throughput of 180 Mbps or higher may be difficult in theory. In order to obtain such a data rate, signals are transmitted through a physical layer by using a high-order modulation scheme such as a 64-QAM scheme at a low code rate such as a 5/6 code rate, and an aggregation or block ACK scheme is applied in order to minimize a header in a media access control (MAC) layer and thus reduce an overhead. However, this scheme is greatly affected by noise in a wireless communication system using multiple antennas.
In a multiple antenna receiver, noise distribution of the multiple antenna receiving paths has great influence on the performance of a detector for detecting signals received through multiple antennas. This is because it is assumed that noise distributions of the multiple antenna receiving paths are uniform in order to minimize the complexity of the detector. If the noise distribution is differently generated among the multiple receiving paths, a more complex algorithm is required for detecting the transmit signals at the receiving end, and a larger amount of hardware resources are used. According to the existing method, in order to equalize the noise distributions among the multiple receiving paths, a received signal is FFT-processed, and a magnitude of a noise signal at an unused frequency is calculated. In this way, noises of the multiple receiving ends are calculated, and its difference is compensated. However, this method must have complex hardware in order to calculate exact noise. Furthermore, when hardware is made in a simple structure, its accuracy is lowered, resulting in the degradation of system performance. Moreover, since a plurality of noise figures are not a time-varying function but is derived from a noise figure of an antenna and an analog device, the method which calculates a noise distribution at each time and reflects the calculated noise distribution may cause the performance degradation due to an estimation error. Consequently, there is a need for a modulator which has a simple hardware structure and a low complexity.